linux-kernel - Re: [PATCH v8 7/7] x86/crash: Add x86 crash hotplug support for kexec

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Date:   Wed, 25 May 2022 19:56:39 +0530
From:   Sourabh Jain <sourabhjain@...ux.ibm.com>
To:     Eric DeVolder <eric.devolder@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v8 7/7] x86/crash: Add x86 crash hotplug support for
 kexec_load

Hello Eric,

On 06/05/22 00:16, Eric DeVolder wrote:
> For kexec_file_load support, the loading of the crash kernel occurs
> entirely within the kernel, and as such the elfcorehdr is readily
> identified (so that it can be modified upon hotplug events).
>
> This change enables support for kexec_load by identifying the
> elfcorehdr segment in the arch_crash_handle_hotplug_event(),
> if it has not already been identified.
>
> Signed-off-by: Eric DeVolder <eric.devolder@...cle.com>
> Acked-by: Baoquan He <bhe@...hat.com>
> ---
>   arch/x86/kernel/crash.c | 24 ++++++++++++++++++++++++
>   1 file changed, 24 insertions(+)
>
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 951ef365f0a7..845d7c77854d 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -485,6 +485,30 @@ void arch_crash_handle_hotplug_event(struct kimage *image,
>   	void *elfbuf = NULL;
>   	unsigned long mem, memsz;
>   
> +	/*
> +	 * When the struct kimage is alloced, it is wiped to zero, so
> +	 * the elfcorehdr_index_valid defaults to false. It is set on the
> +	 * kexec_file_load path, or here for kexec_load, if not already
> +	 * identified.
> +	 */
> +	if (!image->elfcorehdr_index_valid) {
> +		unsigned int n;
> +
> +		for (n = 0; n < image->nr_segments; n++) {
> +			mem = image->segment[n].mem;
> +			memsz = image->segment[n].memsz;
> +			ptr = map_crash_pages(mem, memsz);
> +			if (ptr) {
> +				/* The segment containing elfcorehdr */
> +				if (memcmp(ptr, ELFMAG, SELFMAG) == 0) {
> +					image->elfcorehdr_index = (int)n;
> +					image->elfcorehdr_index_valid = true;

How about finding elfcorehdr index on kexec_load path post 
kimage_load_segment call in
do_kexec_load (kernel/kexec.c) or other suitable place? This way we can 
avoid checking for
elfcorehdr index for every hotplug. Also we might not need 
image->elfcorehdr_index_valid.

Thanks,
Sourabh Jain